An acoustic echo canceller employing linear echo processing estimates a transfer function of an echo path from a speaker to a microphone by means of an adaptive filter or the like, and generates a pseudo-echo signal. Then, it eliminates echo contained in transmission voice by subtracting the pseudo-echo signal from a transmission voice signal input to the microphone. The acoustic echo canceller has an advantage of being able to eliminate only the echo without impairing the transmission voice even in a state in which the echo and transmission voice overlap each other, that is, in a double talk state. However, because of an estimation error that can occur in the transfer function of the echo path and a residual echo that can occur owing to nonlinear echo and the like, the resultant echo canceling effect has its own limit.
On the other hand, an acoustic echo canceller using nonlinear echo processing suppresses echo directly by applying amplitude suppression processing to a voice signal using a voice switch, center clipper and variable attenuator. The acoustic echo canceller has an advantage of being able to eliminate echo surely without producing residual echo with a simple configuration. However, it has a problem of impairing quality of sound because it suppresses transmission voice together with the echo in a double talk state.
To solve the foregoing problems, a standard acoustic echo canceller eliminates a main component of the echo by linear echo processing, and eliminates residual echo after the elimination by the nonlinear echo processing only when the transmission voice is inactive. Such a configuration is described in a Non-Patent Document 1.
However, in an environment in which a transmission-voice-to-echo ratio is comparatively bad in the signal power, since the transmission voice is tend to be hidden easily by the echo, it becomes harder to make a decision as to whether the transmission voice is active or not. As a result, the acoustic echo apparatus cannot detect the transmission voice in the double talk state. Accordingly, it has a problem of impairing the transmission voice by carrying out excessive nonlinear echo processing.
To solve the foregoing problem, Patent Document 1, for example, discloses a method of flexibly controlling a suppression coefficient used for the amplitude suppression of the voice signal in the nonlinear echo processing.
An echo suppressor disclosed in Patent Document 1 computes the power of only the transmission voice by subtracting the power of the echo signal from the power of the transmission signal including the transmission voice and echo superposed thereon, adds a prescribed masking threshold to the transmission voice power, and employs the value normalized by the power value of the transmission signal as the suppression coefficient. By carrying out the nonlinear echo processing by using the suppression coefficient, the echo suppressor automatically weakens the suppression in the double talk state in which the transmission voice is active, thereby making it harder to impair the transmission voice. Incidentally, the power of the echo signal is estimated from the product of the power of the received signal and the amount of acoustic coupling of the echo path.
Non-Patent Document 1: ITU-T Recommendation G.165 (pp. 17-20).
Patent Document 1: Japanese Patent Laid-Open No. 2002-84212 (Paragraphs 0039-0042).
For example, although a conventional echo canceller such as that described in Patent Document 1 estimates the power of the echo signal from the product of the power of the received signal and the amount of acoustic coupling of the echo path, it is difficult to measure the amount of acoustic coupling because it is subjected to the influence of fluctuations of the echo path and surrounding noise. Accordingly, the estimated power of the echo signal always has an estimation error, and hence the transmission voice power computed from the power of the echo signal has an error.
In particular, in an environment in which the transmission-voice-to-echo ratio is very bad such as a hands-free phone, the influence of the error is conspicuous. Thus, it sometimes occurs that the estimation value of the transmission voice power becomes below zero when computing the power of the transmission signal by subtracting the estimation value of the power of the echo signal from the power of the transmission signal. As a result, a problem arises of impairing the transmission voice because of excessive echo suppression occurring in the nonlinear echo processing.
With the foregoing configuration, the conventional echo canceller has a problem of impairing the transmission voice in the double talk state in the environment in which the transmission-voice-to-echo ratio is bad.
Incidentally, to avoid the problem of impairing the transmission voice in the conventional method described by way of example of Patent Document 1, the masking threshold described before must be set at a very high value to reduce the echo suppression. In this case, however, a problem arises in that the echo cannot be suppressed sufficiently as a harmful influence.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide an echo canceller capable of achieving sufficient echo processing performance without impairing the transmission voice even in a double talk state in an environment in which the transmission-voice-to-echo ratio is bad.